This invention relates to an auto-tensioner used mainly to maintain tension of a timing chain or belt for driving camshafts of an engine.
In a typical engine, the rotation of its crankshaft is transmitted to camshafts through a timing chain or belt (the following description is made for a timing chain only) received in an engine cover to rotate the camshafts, thereby opening and closing valves mounted to combustion chambers. In order to keep the tension of the chain within a proper range, a tension adjusting device is typically used which comprises a chain guide pivotable about a shaft, and a chain tensioner for pressing the chain guide against the chain.
Among chain tensioners mounted in such tension adjusting devices, one as disclosed in JP utility model publication 64-41755 is known which includes a cylindrical housing having open and closed ends, a rod axially slidably inserted in the housing, an oil seal mounted in the housing at its open end for sealing oil in the housing, a return spring mounted in the housing to bias the rod in a direction away from the closed bottom of the housing, a screw rod mounted in the housing and having an external thread on its outer periphery which is in threaded engagement with an internal thread formed on the inner periphery of the rod at its end portion, and a spring mounted between the screw rod and the rod and biasing the screw rod away from the rod.
While the camshafts are being driven with the chain vibrating, when the tension of the chain increases, the rod repeatedly moves back and forth within the range of the gap between the internal and external threads, with the screw rod slowly rotating. The rod thus axially moves to a position where the biasing force of the return spring balances with the tension of the chain, thereby reducing the tension of the chain.
When the tension of the chain decreases, the rod moves in a direction away from the bottom of the housing until the force of the return spring balances with the tension of the chain. When the rod moves in this direction by a predetermined distance, the screw rod momentarily separates from the bottom of the housing. But soon thereafter, the screw rod moves axially while turning under the force of the spring, and abuts the bottom of the housing again.
When the engine is stopped, the screw rod remains in abutment with the bottom of the housing. In this state, although the tension of the chain may be greater than the biasing force of the return spring, because the chain does not vibrate, the screw rod never rotates, so that the rod does not move toward the bottom of the housing. Thus, when the engine is restarted, the tensioner can quickly remove any slackness of the chain, so that the tension of the chain can be kept stable.
In this auto-tensioner, because the internal thread which threadedly engages the external thread of the screw rod is directly formed on the rod, if a force acts on the end of the rod protruding from the housing in a direction perpendicular to the axis of the rod, the axes of the internal and external threads tend to be displaced from each other, which in turn increases the resistance to rotation of the screw rod relative to the rod.
Thus, if a force is applied to the end of the rod protruding from the housing in a direction perpendicular to the axis of the rod (such a force is applied if e.g. the chain guide pivots due to increased tension of the chain, so that the contact point between the chain guide and the rod moves), the resistance to rotation of the screw rod relative to the rod increases, so that it may become difficult to sufficiently reduce fluctuations in tension of the chain.
Another problem with this auto-tensioner is that because the rod cannot move sufficiently quickly when the tension of the chain increases, if the tension of the chain changes sharply while the engine is running (due e.g. to resonance of the engine or large fluctuations in torque of the crankshaft per combustion cycle), it is impossible to sufficiently reduce the tension of the chain. Thus, the tension of the chain may increase excessively while the engine is running.
An object of the present invention is to provide an auto-tensioner which operates stably even if a force is applied to the end of the rod protruding from the housing in a direction perpendicular to the rod axis.